Browsing by Author "Wu, TM"

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A possible nonpolar solvation mechanism at an intermediate time scale: the solvent-cage expansion
(2000-07-14)
Chang, SL
;
Wu, TM
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物理研究所
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Institute of Physics
Through studying the so-called 'epsilon' nonpolar solvation model, we propose that the expansion of the solvent cage, which is composed of the solvents at the vicinity of the first maximum of the solute-solvent radial distribution function, might be a possible solvation mechanism at a time scale between inertial motions and structural relaxation. The expansion is triggered by the strong repulsion between the solute and its neighboring solvents, originally inside the cage. This intermediate mechanism is found to be the cause for the bump following the ultrafast decay in the solvation time correlation functions of the 'epsilon' model at low densities. (C) 2000 Elsevier Science B.V. All rights reserved.
Characteristics of instantaneous resonant modes in simple dense fluids with short-ranged repulsive interactions
(2000-07-01)
Wu, TM
;
Ma, WJ
;
Chang, SL
;
物理研究所
;
Institute of Physics
We manifest the characteristics of the low-frequency, quasilocalized instantaneous normal modes, named as the instantaneous resonant modes (IRMs), in simple dense fluids with short-ranged repulsive interactions. The analyses include the potential energy profiles of the IRMs, and the local geometric structures and the number of the interacting neighbors of the particles at which the centers of the quasilocalization are located. We conclude that an IRM is created due to fluctuations in the local density, and has a barely-isolated center, which slightly interacts with one or two nearest neighbors, and the potential energy profile of an IRM is basically single-well with strong anharmonicity. The differences in character between the IRMs and the high-frequency localized instantaneous normal modes are also examined. Based on the barely isolated center picture, a necessary criterion for the occurrence of the IRMs is proposed. While only the imaginary-frequency IRMs are found in dense fluids with purely repulsive interactions satisfying the criterion, a tiny attractive well in the pair potential allows the occurrence of the real-frequency IRMs. The physical systems to detect the presence of the IRMs are discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)50225-6].
Conservation constraints on random matrices
(2003-02-07)
Ma, WJ
;
Wu, TM
;
Hsieh, J
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物理研究所
;
Institute of Physics
We study the random matrices constrained by the summation rules that are present in the Hessian of the potential energy surface in the instantaneous normal mode calculations, as a result of momentum conservation. In this paper, we analyse the properties related to such conservation constraints in two classes of real symmetric matrices: one with purely row-wise summation rules and the other with the constraints on the blocks of each matrix, which underscores partially the spatial dimension. We show explicitly that the constraints are removable by separating the degrees of freedom of the zero-eigenvalue modes. The non-spectral degrees of freedom under the constraints can be realized in terms of the ordinary constraint-free orthogonal symmetry but with the rank deducted by the block dimension. We propose that the ensemble of real symmetric matrices with full randomness, constrained by the summation rules, is equivalent to the Gaussian orthogonal ensemble (GOE) with lowered rank. Independent of the joint probability distribution, the Jacobian contributed by the transformation from the matrix coordinates to the spectral coordinates, possesses the same spectral-variable dependence in these two cases. We verify numerically that the small-separation asymptotic in the nearest-neighbour level spacing distribution is indeed governed by the same symmetry power factor, beta = 1, as that of the GOE, under conservation constraints and other underlying spatial correlations.
Dynamics of atomic pairs in a Lennard-Jones fluid: Mean relative displacement analysis
(1999-03-01)
Wu, TM
;
Chang, SL
;
物理研究所
;
Institute of Physics
We have analyzed the dynamics of atomic pairs in a simple fluid in terms of the mean relative displacement of two atoms with a given initial separation. The short-time expansion of the mean relative displacement has been derived exactly beyond the constant acceleration approximation (CAA) (up to the t(4) order). For a Lennard-Jones fluid, the mean relative displacements calculated by either the CAA or the t(4)-order approximation have been compared with the results obtained from a molecular-dynamics simulation. Through studying the relative motions of atomic pairs with different initial separations, we give a physical picture of the dynamics of neighboring atoms beyond the CAA. [S1063-651X(99)08603-1].
Electron-phonon scattering times in three-dimensional disordered Sb films
(2000-02-01)
Lin, JJ
;
Li, TJ
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Wu, TM
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電子物理學系
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物理研究所
We have measured the electron-phonon scattering times t(ep) in a series of three-dimensional Sb films having the characteristic of ql similar to 1, where q is the wave number of the thermal phonons, and I is the electron elastic mean free path. We observe that 1/t(ep) similar to T-p, With P approximate to 2.4, in the temperature range 1-14 K. In addition, we find a very weak dependence of 1/t(ep) on l. Our observations are compared with theoretical predictions for electronphonon interactions in the presence of disorder and with previous experimental results in thin Sb films.
Evidence for instantaneous resonant modes in dense fluids with repulsive Lennard-Jones force
(1999-01-01)
Wu, TM
;
Ma, WJ
;
物理研究所
;
Institute of Physics
In terms of instantaneous-normal-mode (INM) analysis and a newly defined measure for quasilocalization, we present the evidence for the resonant modes in a model fluid, in which the pair interaction is merely the repulsive portion of the Lennard-Jones potential. We name such a quasilocalized INM as an instantaneous resonant mode (IRM). By examining the potential energy profile beyond the INM approximation, we conclude that the IRMs occur in single-well potentials with strong enough anharmonicity. (C) 1999 American Institute of Physics. [S0021-9606(99)50301-2].
Evidence for instantaneous resonant modes in dense fluids with repulsive Lennard-Jones force
(1999-01-01)
Wu, TM
;
Ma, WJ
;
物理研究所
;
Institute of Physics
In terms of instantaneous-normal-mode (INM) analysis and a newly defined measure for quasilocalization, we present the evidence for the resonant modes in a model fluid, in which the pair interaction is merely the repulsive portion of the Lennard-Jones potential. We name such a quasilocalized INM as an instantaneous resonant mode (IRM). By examining the potential energy profile beyond the INM approximation, we conclude that the IRMs occur in single-well potentials with strong enough anharmonicity. (C) 1999 American Institute of Physics. [S0021-9606(99)50301-2].
Instantaneous normal mode analysis of liquid Na
(1996-11-22)
Wu, TM
;
Tsay, SF
;
物理研究所
;
Institute of Physics
The instantaneous-normal-mode (INM) densities of states for liquid Na in four realistic thermodynamic states are obtained by molecular-dynamics simulation. The exponent functions derived from the INM densities of states are found to fit well with a binomial of frequency for the imaginary-frequency lobe and a three-term polynomial for the real-frequency lobe. Through the study of the size dependence for the participation ratios of INMs, we show that for both the real-frequency and the imaginary-frequency lobes, INMs of low frequencies are extended modes and INMs in the high-frequency end are localized modes. However, each lobe has a broad transition region in frequency from the extended to localized INMs. (C) 1996 American Institute of Physics.
Instantaneous normal mode analysis of liquid Na
(1996-11-22)
Wu, TM
;
Tsay, SF
;
物理研究所
;
Institute of Physics
The instantaneous-normal-mode (INM) densities of states for liquid Na in four realistic thermodynamic states are obtained by molecular-dynamics simulation. The exponent functions derived from the INM densities of states are found to fit well with a binomial of frequency for the imaginary-frequency lobe and a three-term polynomial for the real-frequency lobe. Through the study of the size dependence for the participation ratios of INMs, we show that for both the real-frequency and the imaginary-frequency lobes, INMs of low frequencies are extended modes and INMs in the high-frequency end are localized modes. However, each lobe has a broad transition region in frequency from the extended to localized INMs. (C) 1996 American Institute of Physics.
Instantaneous normal mode analysis of orientational motions in liquid water: Local structural effects
(2004-08-22)
Chang, SL
;
Wu, TM
;
Mou, CY
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物理研究所
;
Institute of Physics
We have investigated the effects of local structures on the orientational motions in liquid water in terms of the instantaneous normal mode (INM) analysis. The local structures of a molecule in liquid water are characterized by two different kinds of index: the asphericity parameter of its Voronoi polyhedron and the numbers of the H bonds donated and accepted by the molecule. According to the two kinds of index, the molecules in the simulated water are classified into subensembles, for which the rotational contributions to the INM spectrum are calculated. Our results indicate that by increasing the asphericity, the rotational contribution has a shift toward the high-frequency end in the real spectrum and a decrease in the fraction of the imaginary modes. Furthermore, we find that this shift essentially relies on the number of the donated H bonds of a molecule, but has almost nothing to do with that of the accepted H bonds. The local structural effects resulting from the geometry of water molecule are also discussed. (C) 2004 American Institute of Physics.
Instantaneous resonant modes in high-temperature gallium liquids
(2001-08-01)
Wu, TM
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Tsay, SF
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Chang, SL
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Ma, WJ
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物理研究所
We have studied the low-frequency, quasilocalized vibrational excitations in Ga liquids in terms of instantaneous-normal-mode analysis. We present strong numerical evidence for the existence of the instantaneous resonant modes (IRM's) in high-temperature Ga liquids. It is the exotic repulsive-core interaction between Ga particles that gives rise to the IRM's, which are associated with special local structures created by density fluctuation. In general, the IRM density of states is found to increase with temperature.
Level statistics of Hessian matrices: random matrices with conservation constraints
(2003-04-01)
Ma, WJ
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Wu, TM
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Hsieh, J
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Chang, SL
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物理研究所
We consider the Hessian matrices of simple liquid systems as a new type of random matrices. By numerically comparing the distribution of the nearest-neighbor level spacing of the eigenvalues with the Wigner's surmise, we round that the level statistics is akin to the generic Gaussian Orthogonal Ensemble (GOE), in spite of the constraints due to the summation rules and the presence of the correlation among the components inherited with the underlying spatial configuration. The distribution is in good agreement with the Wigner's prediction if only the extended eigenstates are considered. Indeed, our theoretical analysis shows that the ensemble of real symmetric matrices with full randomness. but constrained by the summation rules, is equivalent to the GOE with matrices of the rank lowered by the spatial dimension. (C) 2002 Published by Elsevier Science B.V.
Local geometric structures of instantaneous resonant modes in Ga liquids
(2002-05-01)
Wu, TM
;
Ma, WJ
;
Chang, SL
;
Tsay, SF
;
物理研究所
Recently, we have numerically found the instantaneous resonant modes (IRMs) in high-temperature Ga liquids. The occurrence of these IRMs is attributed to the cooperation of the exotic repulsive core of the Ga pseudopotential and the local volume expansion. In this paper, the local structures around the quasilocalized centers of these IRMs are analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.
Local structural effects on low-frequency vibrational spectrum of liquid water: The instantaneous-normal-mode analysis
(2006-01-10)
Tsai, KH
;
Wu, TM
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物理研究所
;
Institute of Physics
Currently, the designations for the low-frequency vibrational spectrum of liquid water are still diversified. In this Letter. using the instantaneous normal mode (INM) formalism for subensembles, we have examined the effects of local structure on the low-frequency INM spectrum of liquid water. The classified subensembles of water molecules are characterized by either the geometry of Voronoi polyhedron or the H-bond configuration. With these subensembles, the translational contribution to the INM spectrum is investigated. Providing insights into the local structural dependence of the vibrations between H-bonded water molecules. our results are consistent with the conventional designations for the Raman spectrum of liquid water. (c) 2005 Elsevier B.V. All rights reserved.
Localized instantaneous normal modes in liquid Na
(1997)
Wu, TM
;
Tsay, SF
;
物理研究所
;
Institute of Physics
The instantaneous-normal-mode (INM) densities of states for liquid Na in four realistic thermodynamic states are obtained by molecular-dynamic simulation. Through the study of the size dependence for the average number of the excited particles in an INM, rye show that the INMs in the high-frequency end of each lobe are localized modes. By examining the projection components of the eigenvector of a localized INM on each particle, we show that the localized INM indeed has only one excited center at the particle with the largest projection component and the spatial size of the localization extends to the first nearest neighbor shell from the central particle. We also give a scenario of how a localized INM becomes an extended INM as the frequency decreases.
Mechanism for singular behavior in vibrational spectra of topologically disordered systems: Short-range attractions
(2005-05-22)
Wu, TM
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Chang, SL
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Tsai, KH
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物理研究所
;
Institute of Physics
At low-enough fluid densities, we have found some naive singular behavior, like the van Hove singularities in the phonon spectra of lattices, appearing in the instantaneous normal mode spectra of the Lennard-Jones (LJ) 2n-n fluids, which serve as a prototype of topologically disordered systems. The singular behavior cannot be predicted by the mean-field theory, but interpreted by the perturbed binary modes of some special pairs, called the mutual nearest neighbor pairs, at separations corresponding to the extreme binary frequencies, which are solely determined by the attractive part of the LJ 2n-n pair potential. By reducing the range of attraction in the pair potential under the conditions of the same particle diameter and well depth, the tendency for the appearance of the singular behavior shifts to higher fluid densities. From this study, we conclude that pair potential with a short-range attraction can be a mechanism to produce a counterpart of the van Hove singularity in the vibrational spectra of disordered systems without a reference lattice. 2005 American Institute of Physics.
Microscopic morphology and energy surface landscape in a supercooled soft-sphere system
(2000-06-15)
Ma, WJ
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Wu, TM
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物理研究所
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Institute of Physics
We carry out a systematic study on the microscopic morphology and its relationship with the signatures of the traces swept over the potential energy surface in a soft-sphere fluid system interacting via a truncated Lennard-Jones potential. The state of the model system is tuned by changing its temperature obtained in a molecular dynamics simulation, ranging from the equilibrium fluid phase to the deeply quenched supercooled states. The density and the range of interaction are chosen so that the instantaneously barely isolated centers are present. We analyze the microscopic structural origin of the features in the instantaneous-normal-mode (INM) spectrum and search for evidence of state transformation. It is found that the presence of drastic changes in the INM spectra upon entering the deep supercooled regime is accompanied by the reduction in the spatial connectivity. We develop a method to characterize the spatial configuration ration in an attempt to address the origin of the temperature-dependent changes in the spectra. (C) 2000 Elsevier Science B.V. All rights reserved.
The instantaneous-normal-mode analysis on the potential energy landscapes of the Lennard-Jones 2n-n fluids
(2004)
Wu, TM
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Chang, SL
;
物理研究所
;
Institute of Physics
We have examined the instantaneous-normal-mode spectra of the Lennard-Jones 2n-n fluids with an aim to characterize the potential energy landscape (PEL) of a simple system. Under the isothermal-isochoric ensemble, the fluid density and the value of n are two independent parameters to control the PEL. We perform two ways of investigation by changing the value of n at a fixed fluid density and temperature and varying the fluid density for several different n values.